Position measuring devices are widely used in tool machines and automation systems, in particular. They are employed for determining the relative position of two objects that can move relative to each other. A basic distinction is made between linear and angle measuring devices. Linear measuring devices, for example, are used to determine the relative position of two machine components of a tool machine that are able to move with respect to each other. A measuring standard, e.g., in the form of a straight scale on which a graduation is provided, is connected to one of the two objects for this purpose, and a scanning unit is connected to the other, so that position-dependent scanning signals, which can be used to determine the degree of movement of the two objects with respect to each other along the movement direction, can be obtained by scanning the graduation.
Angle-measuring devices, also referred to as rotary encoders, are designed according to the same principle. Instead of the scale, however, a circular disk is used as a measuring standard in such a case, on which the graduation is applied concentrically with the pivot point. The disk is connected in rotationally fixed manner to a shaft to be measured, while the scanning unit is fixedly mounted with respect thereto.
The graduation may include one or more graduation track(s), a distinction being made between incremental and absolute encoded graduation tracks. Incremental graduation tracks are made up of evenly spaced graduation elements, which, given a uniform relative movement between measuring standard and scanning unit, provide substantially sinusoidal scanning signals when scanned. In this case, the travel is ascertained by counting the signal periods or fractions of signal periods. An absolute travel determination requires the specification of a reference point, for which a reference mark is provided, for example, which is likewise detected by the scanning unit. The scanning signals from absolute encoded graduation tracks, on the other hand, are able to be used for a direct detection of the absolute position, inasmuch as they are encoded in multiple adjacently located graduation tracks (e.g., Gray code, BCD code, etc.) or are serially encoded in one graduation track (e.g., chain code, PRC). In addition, there are also graduations that include both incremental and absolute graduation tracks.
One widely used operating principle in the case of position measuring devices is optical scanning. In this case, directed light that is emitted by a light source is used for imaging the graduation mounted on a measuring standard on a number of photodetectors. The measuring standard is disposed in the optical path of the light in a manner that allows it to move, and modulates the light when the graduation is moved in relation to the light source and the photodetectors. The position, or the change in position, is ascertained by analyzing the output signals of the photodetectors. The graduation may consist of regions having different optical characteristics such as transparent/opaque.
The photodetectors required for the scanning are usually situated together on a semiconductor chip, which is mounted directly on a circuit board using chip-on-board technology (COB). In the process, the rear side of the chip is first glued to the circuit board and the contact surfaces on its topside are then connected to the circuit board using wire bonding. In addition to the photodetectors, at least parts of the evaluation electronics for the scanning signals are frequently situated on the semiconductor chip as well.
FIG. 3 schematically illustrates the components that are relevant in connection with the position measurement of a conventional position measuring device, i.e., a scanning unit 10 and a measuring standard 40, which includes a graduation 41.
Scanning unit 10 and measuring standard 40 are situated in a manner that allows them to move relative to each other in a measuring direction X. For example, if the position measuring device is a device for a linear measurement (linear position measuring device), then measuring standard 40 is provided in the form of a straight scale, which is mounted on a first machine component and on which a graduation 41 is present in measuring direction X. Scanning unit 10 is fastened to a second machine component, such that when the first machine component executes a movement in measuring direction X in relation to the second machine component, scanning unit 10 is moved along the scale and scans graduation 41 in the process. The scanning results in scanning signals, from which position values are generated in the course of further processing.
In the case of rotary position measuring devices (rotary transducers or angle-measuring devices), measuring standard 40, for example, is a circular disk, which is connected to a shaft whose angular position is to be measured in a rotatably fixed manner. Graduation 41 is disposed concentrically with the rotational axis of the shaft. The scanning unit, on the other hand, is firmly mounted such that the graduation can be scanned when the shaft to be measured executes a rotation, and position values, in this case, angle values, are in turn able to be generated from the scanning signals.
Scanning unit 10 includes an illumination unit 20 and a detector unit 530. The operating principle on which the position measuring device is based is optical transmitted-light scanning. This means that graduation 41 on measuring standard 40 is positioned between illumination unit 20 and detector unit 530. Illumination unit 20 emits light in the direction of graduation 41. Graduation 41 includes light-transmitting (transparent) and opaque regions, which modulate the light of illumination unit 20. The modulated light is imaged on detector unit 530, which then generates positional signals from it.
Optical transmitted-light scanning uses light that is collimated. To generate collimated light, illumination unit 20, for instance, may include a light source 21 which radiates divergent light that is collimated by a collimator 22.
Measuring standard 40 may be made from a transparent material, such as glass. In this case, graduation 41 is formed by opaque regions, for instance, made of metal that is applied on the measuring standard. Chromium, for example, is especially suitable for this purpose. However, there are also measuring standards 40 which are made from an opaque material, such as metal. Graduation 41 may include a sequence of metal webs and openings in measuring standard 40.
Detector unit 530 includes a circuit board 531, a sensor unit 532, and, in particular when small graduation periods of graduation 41 are to be scanned, a scanning plate 533.
Circuit board 531 functions, for example, as a carrier for sensor unit 532, which constitutes the central component of detector unit 530. In addition to supplementary electronic circuits for sensor unit 532, input/output interfaces in the form of plug-and-socket connectors, for instance, are provided on circuit board 531.
Sensor unit 532 is arranged as a semiconductor chip. It has a front side and a rear side, the side facing circuit board 531 forming the rear side, and the side facing graduation 41 forming the front side of sensor unit 532. Situated on the front side of sensor unit 532 are a number of photodetectors 535, which are used for generating positional signals by detecting the light of illumination unit 20, which is modulated by graduation 41 and possibly scanning plate 533. The front side of sensor unit 532 is preferably aligned in parallel with the plane in which graduation 41 is situated. In addition to photodetectors 535, sensor unit 532 includes additional components for processing the positional signals, possibly even including the generation of position values from the positional signals. Moreover, the sensor unit may include a multitude of additional circuit blocks, such as fault correction, circuits for ensuring functional safety, and a digital interface for communicating with sequential electronics. A semiconductor chip which includes both photodetectors and components for signal processing is referred to as an opto-ASIC.
The rear side of sensor unit 532 is connected to circuit board 531, the connection being created by bonding, for example. For the electrical connection of the circuit components of sensor unit 532 to circuit board 531, the front side of sensor unit 532 and the side of circuit board 531 facing sensor unit 532 are provided with corresponding contact surfaces, which are connected to each other in a conventional manner by wire bonding using bonding wires 537. An encapsulating material 538 protects bonding wires 537 from mechanical effects.
Wire bonding is a complex manufacturing process, both in terms of the purchasing costs of the required manufacturing systems (wire bonding) and the additional time required for the wire bonding process, the encapsulation of the bonding wires and curing of the encapsulation mass.
The result of this packaging is a dam around the photodetectors, which projects by a height c in relation to the front side of sensor unit 532 (i.e., the surface of the semiconductor chip on which the photodetectors are situated). Height c amounts at least to 0.5 mm in conventional production methods. Taking into account manufacturing tolerances attributable in particular to fluctuations in the thickness of the encapsulation mass 538, a scanning distance d of between 0.55 mm and 0.7 mm must be maintained in order to be able to ensure a safe operation, this scanning distance d being defined by the clearance between the front side of sensor unit 32 (the surface of the semiconductor chip on which the photodetectors are situated) and the surface of measuring standard 40 (which is to be equated with the surface of graduation 41 in the example illustrated).
Scanning plate 533 is required in particular when scanning small graduation periods. It is situated between photodetectors 535 on the front side of sensor unit 532 and graduation 41. Similar to measuring standard 40, it has a graduation structure that optimizes the imaging of graduation 41 to photodetectors 535.
German Published Patent Application No. 198 55 307 describes a scanning unit in which the sensor unit is contacted with the aid of wire bonding. A cover element, the thickness of which is even greater than that of the contacting region, is additionally situated above the radiation-sensitive region.
Any dirt that accumulates in the optical path of the light has a detrimental effect on the function in the optical scanning principle. This applies to liquid contaminants, in particular. It occurs in the form of fluid droplets, which consist of lubricants, condensed water, coolant, etc., for example. Not only does this reduce the analyzable light quantity that impinges upon the photodetectors, but it also disperses the light, so that it is distributed to multiple photodetectors situated next to each other.
The dam of encapsulating material 538 has negative effects with regard to dirt deposits, since it promotes the collection of dirt deposits within the dam, depending on the position.
To prevent problems caused by dirt deposits, special attention is paid during the construction process of position measuring devices featuring optical scanning to ensure that the introduction of dirt particles, whether in solid or liquid form, is avoided. For example, this may be accomplished through the material selection or special constructive measures.
However, it is difficult to avoid contamination altogether. Therefore, an attempt is made to compensate for the effect of dirt particles by creating redundancy in the scanning or by complex signal processing of the scanning signals.
Despite all of these measures, dirt particles, especially in liquid form, are a potential source of failure in optical position measuring devices.